1. Field of the Invention
The invention relates to a differential. More particularly, the invention relates to a front differential in a vehicle that is capable of switching between rear-wheel drive and front- and rear-wheel drive.
2. Description of the Related Art
A differential for a vehicle is widely used. This differential distributes driving force input from an input shaft to left and right driving wheels. Differential operation of the unit enables the left and right wheels to rotate at different speeds. Japanese Utility Model Application Publication No. 61-112154 (JP-U-61-112154) describes one example of a front differential.
Incidentally, some vehicles such as pickup trucks and SUVs (Sport Utility Vehicles) are provided with an ADD unit (Automatic Disconnecting Differential) for the front differential because these vehicles run switching between rear-wheel drive and front- and rear-wheel drive. This ADD unit can place the vehicle in rear-wheel drive by interrupting the drive transmission path between one side gear of the front differential and the left or right front wheel that corresponds to that side gear.
When the ADD unit places the vehicle in rear-wheel drive, there is no disengaging load between the pinion gear and the side gear of the front differential, so it does not help to support the drive shaft. That is, the drive shaft is supported only by the engaged portion in the front differential so the drive shaft ends up leaning. As a result, the side gear also ends up leaning, which may cause misalignment. Also, when the vehicle is in rear-wheel drive, the front differential is constantly in a differential state so the pinion gear that is in mesh with the side gears rotates (i.e., spins) and revolves as the side gears rotate, producing noise and vibration from gear mesh. What is more, if misalignment occurs, the vibratory force from the meshing of the gears will adversely affect the front differential gear unit greatly.